Generally, parasitics in an optical device (e.g., a laser device) refer to phenomena that act to clamp or limit useful gain (e.g., longitudinal gain) within a gain medium of the optical device, for example, due to internal reflections. In many cases, parasitic optical rays undergo total internal reflection (TIR) within the gain medium and continuously loop around and increase in power (robbing useful gain) with virtually no loss. In one conventional approach, parasitic suppression is accomplished by using an E-wave coating directly on the gain medium that limits the numerical aperture (NA) of the gain medium. The E-wave coating must then be processed in some way on its outer surface to prevent parasitic rays from returning back into the gain medium (e.g., roughening the outer surface of the coating). These processes add cost and complexity to the overall design. Roughening the gain medium surface alone (without an E-wave coating) has been tried, but this greatly increases the scattering loss of the guided pump light, which is highly undesirable as it reduces pump energy. Another conventional approach involves use of an optical adhesive with a refractive index sufficient to limit the gain medium NA. Typically, the adhesive is used to secure the gain medium to a heat-sink and therefore, is applied on the cooling surfaces of the gain medium. It is not convenient to use this adhesive on surfaces not involved in cooling or mounting. Another issue with optical adhesives is their high coefficient of thermal expansion (CTE). This property can significantly reduce the alignment stability of the gain medium over ranges of environmental temperature and pump power.
Another conventional device includes a bonded absorbing side cladding that undesirably absorbs pump energy. Yet another conventional arrangement includes doped glass slides between gain elements, but again suffers from the cost and CTE mismatch drawbacks, in addition to increasing processing steps and complexity of the overall optical system. Still yet another conventional arrangement includes providing sensitizer ions in cladding region surrounding the gain medium for absorbing lasing wavelength entering the cladding, to suppress amplified spontaneous emission and parasitic lasing in a plane transverse to the lasing direction. However, this conventional approach still suffers from drawbacks as a substantial portion of the lasing wavelength entering the cladding region is not absorbed due to mismatches in the absorption spectrum of sensitizer ions and the emission spectrum of laser active ions in the gain medium, which are the source of the lasing wavelength, thereby causing parasitics to still exist substantially.